1. Field of the Invention
The invention relates to a gas turbine and a method of cooling a turbine stage of a gas turbine by means of cooling air.
2. Related Art
A gas-turbine plant is normally used to generate electrical energy. In this case, the energy content of a fuel is utilized to produce a rotational movement of a shaft, which in turn drives a generator or a machine. During operation of the gas-turbine plant, ambient air is normally drawn in and compressed to a high pressure in an air compressor assigned to the gas turbine. The greatest proportion of the compressed air passes into one or more combustion chambers and is mixed there with the fuel, for example natural gas.
The gas/air mixture is burned, the hot combustion gases driving the shaft during their expansion to perform work. To this end, the gas turbine comprises a number of turbine blades. The latter are arranged in a plurality of turbine stages comprising guide vanes, which are in a fixed position, and moving blades, which rotate with the rotor. The air compressor of the gas-turbine plant is normally also driven via the rotary movement of the shaft produced in the gas turbine, in addition to the generator.
To increase the performance of the gas turbine and thus achieve as high an efficiency of the gas-turbine plant as possible, an especially high temperature of the combustion gases of, for example, 1000 to 1300xc2x0 C. when entering the gas turbine, is aimed at. However, the upper limit for this temperature results from the thermal loading capacity of the turbine blades.
An increase in the temperature of the combustion gases may be permitted if the turbine blades are cooled in such a way that damage to the gas turbine is reliably prevented. To cool the turbine blades, partial air flows are normally extracted at the air compressor and fed as cooling air to various turbine stages. As a result, however, the air quantity which can be provided for combustion in the gas turbine is also reduced.
The object of the invention is therefore to specify a gas turbine in which the cooling-air quantity provided for cooling a turbine stage is kept small with little technical penalty. In addition, a method of cooling a turbine stage of the gas turbine is to be specified.
The object which relates to the gas turbine is achieved according to the invention by a gas turbine having a turbine stage, comprising a guide vane, and having a feed for cooling air, the feed having a cooling-air outlet, in which the turbine stage can be cooled by the cooling air, which can be fed via the cooling-air outlet to the guide blade, the feed having means by which a predeterminable pressure of the cooling air can be set at the cooling-air outlet. Cooling air can also advantageously be fed to a moving blade of the turbine stage via the guide vane.
With regard to various operating states of the gas turbine, the use of a control unit by which the means of the cooling-air feed can be controlled proves to be advantageous. This control unit measures the pressure at the cooling-air outlet, for example via a measuring line, and then activates the throttle element in accordance with the predetermined pressure, in which case the quantity of cooling air fed can be set.
Means by which the flow velocity and the flow direction of the cooling air can be set downstream of the guide vane are advantageously provided. In this way, the flow velocity and the flow direction of the cooling air, when entering the moving blade and/or the shaft, can be designed in such a way that the cooling air has the local circumferential velocity, in magnitude and direction, of the moving blade and/or of the shaft. In a further advantageous refinement of the invention, these means in this case comprise a swirl nozzle.
The turbine stage into which the cooling-air feed with the cooling-air outlet opens is advantageously a turbine stage which is arranged between a first turbine stage (high-pressure stage) and an end stage (low-pressure stage). With regard to the method of cooling a turbine stage of the gas turbine by means of cooling air, the abovementioned object is achieved according to the invention by virtue of the fact that the cooling air is fed to the guide vane of the turbine stage via a cooling-air outlet connected downstream of a cooling-air feed, a predeterminable pressure of the cooling air being set at the cooling-air outlet. In an advantageous refinement, cooling air is at the same time fed to a moving blade of the turbine stage via the guide vane. In this case, it proves to be advantageous if the flow velocity and flow direction of the cooling air are set on the outlet side downstream of the guide vane.